Electric Forces: IntroductionPhysics 12

Clip of the day:

0 Minute physics!0 http://www.youtube.com/watch?v=9YUtFpLpGfk

Demos:0 Paper and Balloon/comb0 Balloon on wall0 Homemade “Electroscope”0 Electroscope0 Rods (from chem lab)0 Van de Graf generator0 Pith balls

0 http://www.wfu.edu/physics/demolabs/demos/avimov/bychptr/chptr7_electricity.htm

How does an electroscope work?0 An electroscope consists of a vertical metal

rod from which two parallel strips of gold leaf hang down.

0 The gold leaves are enclosed in a glass bottle to protect them from drafts of air.

0 When the metal rod is touched with a conductive charged object the gold leaves spread apart in a V.

0 This is because the charge on the object is conducted through the metal rod to the leaves.

0 Since they receive the same sign charge they repel each other and spread apart.

Electrostatics0 Aka static electricity0 Is the study of electrical forces between stationary or

hardly moving charges0 Defined in terms of its effect (has no colour, mass,

length, etc.)

In everyday life….0 Taking off a sweater over a shirt there

is a crackling sound0 Static cling of clothes out of the dryer0 A pen rubbed with a piece of cloth

will pick up small pieces of paper 0 Cling wrap sticking to everything0 Getting shocked by another person0 Getting a small electric shock from a

cat that has rolled on a synthetic carpet

Chemistry Review???????

0 What is an atom?0 Basic unit of matter that consists of a dense central nucleus

surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons. The electrons of an atom are bound to the nucleus by the electromagnetic force.

0 How can you tell a hydrogen atom from a carbon atom?0 Atoms of each element are distinguished from each other by

the number of protons that are present in their nucleus. An atom containing one proton is a hydrogen atom. An atom containing 6 protons is a carbon atom.

How can an atom orobject be charged?

0 The number of electrons that surround the nucleus will determine whether or not an atom is electrically charged or electrically neutral.

Positively Charged

Negatively Charged

Uncharged

Possesses more

protons than

electrons

Possesses more

electrons than

protons

Equal numbers of protons and

electrons

Why do some things become more easily charged?

0 The presence of different atoms in objects provides different objects with different electrical properties.

0 One such property is known as electron affinity. 0 Electron affinity refers to the relative amount of “love”

that a material has for electrons. 0 High electron affinity, then that material will have a

relatively high “love” for electrons.

Friction: one way to create charge0 The frictional charging process results in a transfer of electrons

between the two objects that are rubbed together. 0 For example:

0 Rubber has a much greater attraction for electrons than animal fur. 0 As a result, the atoms of rubber pull electrons from the atoms of

animal fur, leaving both objects with an imbalance of charge. 0 The rubber balloon has an excess of electrons and the animal fur has

a shortage of electrons. 0 Having an excess of electrons, the rubber balloon is charged

negatively. 0 Similarly, the shortage of electrons on the animal fur leaves it with a

positive charge. 0 The two objects have become charged with opposite types of charges

as a result of the transfer of electrons from the least electron-loving material to the most electron-loving material.

Examples:

0 Higher electron affinity (-)

0 Lower electron affinity (+)

Charging by induction

0 Is a method of charging a neutral object, using a charged object, without establishing physical contact between them.

0 The neutral object becomes polarized0 There is a redistribution of the centers of positive and negative

charges within the object by the movement of electrons across the surface of the object

0 While there are the same number of protons and electrons within the object, these protons and electrons are not distributed in the same proportion across the object's surface.

0 While there is a separation of charge, there is NOT an imbalance of charge. When neutral objects become polarized, they are still neutral objects.

Coulomb’s Experiment0 He used his torsion balance

to measure the magnitudes of the electrical forces between charged objects

0 He was able to prove that an attractive and repulsive force between charges existed and that the magnitude depends on the separation between them.

0 http://www.youtube.com/watch?v=FYSTGX-F1GM

What did he find?0 The electrical force between two charged objects is directly

proportional to the product of the quantity of charge on the objects

0 The electrical force between two charged objects is inversely proportional to the distance of separation between the two objects. 0 Increasing the separation distance between objects decreases

the force of attraction or repulsion between the objects. 0 Decreasing the separation distance between objects increases

the force of attraction or repulsion between the objects.

Coulomb’s Law

0 Fe is electrical force (N)

0 k is the Coulomb constant0 q1 is the electrical charge on

object 1 (C)0 q2 is the electrical charge on

object 2 (C)0 r is distance between the

objects (m)

221

r

qkqFe

Note: Coulomb’s Law0 Determines magnitude of force only

0 Sign is based on charge interactions

0 Electric Force is the force felt by separated (positive or negative) charges.

0 Opposite charges attract0 Like charges repel

Electric Force:

+ -

++

Note: Electrical Force0 Positive and negative does not

indicate direction!

0 Positive indicates a repulsive force 0 Negative indicates an attractive force

Electric Force0 Positive force will result when:

0 Two positively charged particles interact0 Two negatively charged particles interact

0 Negative force will result when:0 One positively and negatively charged particle interact

Conservation of Electrical Charge0 When two charged objects touch, they

transfer charge from one another and the total charge of their system must be conserved.

0 Example: 0 One object with a charge of +4 and another

object with a charge of-12 (total charge is -8). 0 When they touch, the charge is evenly

distributed between each of them (electrons are transferred from the -12 to the +4).

0 When they separate, their total charge still remains -8. Thus, the objects will each have a charge of -4.

Reminder:

0 1 coulomb (C) = 1 000 000 microcoulombs (µC)

Example:0 A positive charge of 6.0 x 10 -6C is 0.30m from a second positive charge of

3.0 x 10 -6C. Calculate the force between the charges.

• Fe = k q1 q2

r2

= (9.0 x 109 N m2/C2 ) (6.0 x 10 -6C) (3.0 x 10 -6C)

( 0.30m )2

= 0.162

0.09

= 1.8 N (repulsion so + force)

Try it :

0 Coulomb’s Law WS#10 Page 638

0 Questions 1-5

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